Known fluoran compounds relating to the present invention are found in Examined Japanese Patent Publication No. 23204/76, No. 29180/76, No. 52759/81 and Unexamined Japanese Patent Publication No. 162690/81. The fluoran compounds disclosed in these gazettes are used as color formers (dye precursors) for heat-sensitive recording sheets, heat-sensitive recording sheets by applying an electric current, pressure-sensitive copying paper, etc.
However, the heat-sensitive recording sheets using these fluoran compounds as color formers requires a high temperature necessary to obtain a color density of 1.0, namely, requires a great consumption energy for attaining a practical color density. In addition, the fluoran compounds encounter disadvantages in moisture resistance, decolorization, oil resistance, resistance to fogging in the background, etc.